Why did the bear cross the road?

A study published today in Proceedings of the Royal Society B shows how wildlife crossings of the Trans-Canada Highway have helped bears find mates on the other side of the road.

Roads connect human populations across vast distances but they can have an adverse effect on the populations of wildlife. Animals are sometimes hit and killed by vehicles and noisy traffic can also deter animals from approaching busy highways. Separating groups on either side of the road reduces their access to mates and so decreases opportunity for genetic mixing. By dividing habitats major roads can isolate small populations, leaving fragmented populations at risk of inbreeding and with limited genetic scope to adapt to changing environments.

To counteract this fragmenting effect underpasses and overpasses have been built along major roads including the Trans-Canada Highway, Canada’s primary east-west transportation route. The highway runs through Banff, Canada’s oldest national park which is home to an array of wildlife, including two species of bear, and is visited by 4 million tourists each year.  Wildlife crossing structures were built in the 1980s and the 1990s to reduce the number of animal-vehicle collisions and to help wildlife cross the fenced off road. Despite such initiatives there is little research to show if these overpasses and underpasses help prevent the isolation of animal populations. Track pads and remote cameras prove that wildlife use crossings but this study, led by Michael Sawaya, is the first to quantify gene flow across the road taking into account migration, successful reproduction and the genetic mixing of populations from north and south of the highway.

The scientists behind the study, which was initiated in 2006, aimed to work out how the highway and the structures crossing it were affecting local grizzly and black bear populations. The study in Banff’s Bow Valley, spanned 3 years.  The researchers studied 20 of the 25 bear crossings along the Trans-Canada Highway using hair-snagging traps. At crossing sites two stretches of barbed wires- parallel to each other at 30 and 70cm from the ground enabled the team to collect hair from bears climbing between them to use the crossing. Away from the road hair was collected with hair traps and bear rubs. This combination of snagging hair at the crossings and away from the road let the team characterise the bears north and south of the highway and identify the individuals crossing the road. The team carried out parentage analysis to help them figure out whether bears who crossed the road were successfully breeding and so work out whether the crossing structures were enabling the flow of genes from one side of the road to the other.

Collecting 1000s of hair samples over their three year study the team were able to identify hundred of bears including 15 grizzlies and 17 black bears who crossed, sometimes frequently, the Trans-Canada Highway. The team found that bear populations were not isolated on either side of the road and that male and female bears from both species were using crossings to successful migrate, breed and carry genes over the road. The researchers’ analysis showed there was no distinction between the population of black bears seen north or south of the road, suggesting that there is completed genetic admixture. In fact, some black bears were frequently spotted crossing the road where they were successfully breeding, siring children on both the north and south sides.

In contrast, though grizzly bear populations weren’t isolated, the team’s genetic characterisation showed that the populations north and south of the road were separate groups, showing that the Trans-Canada highway had had some fragmenting effects on the local grizzly population.  Despite these genetic differences, the team found that grizzly bears as well as black bears were successfully using the crossings to traverse the road and breed on either side. The team concluded that the wildlife crossing structures may be helping to ‘counteract the effects of fragmentation’ across the Trans-Canada Highway.  The scientists concluded that their study has demonstrated that wildlife crossing structures bridging and tunnelling under the Trans-Canada Highway are successfully enabling gene flow in both directions and helping bears in the protected Banff National park to find mates on the other side of the road.